High frequency semiconductor devices such as RF devices are well known in the art. Many high frequency packages employing such devices commonly include an insulating substrate on which the semiconductor is actually mounted. The insulating substrate often includes a conductive pattern therein to which the semiconductor die is electrically coupled, commonly by wire bonds.
Berylium oxide (BeO) is a well known ceramic material often employed as an insulating substrate in RF devices. Berylium oxide is desirable due to its great thermal conductivity properties. However, berylium oxide is very expensive and difficult to use. Further, berylium oxide is toxic in its powdered form and this toxicity has resulted in increased government regulation dealing with its use. RF packages employing ceramics such as berylium oxide are also relatively difficult to frequency match. The problems associated with berylium oxide have led those of skill in the art to look to other insulating materials to employ as substrates in high frequency devices.
Aluminum oxide (Al.sub.2 O.sub.3) has been investigated as a substrate material for RF packages. Although aluminum oxide has many good properties, its thermal conductivity properties are poor and therefore it undesirable for various high frequency applications.
Aluminum nitride substrates have been used in the past. Historically, aluminum nitride substrates are employed with thick film processes. One such thick film process employs screening on a molybdenum/manganese film and then firing it for adhesion. This film typically has a thickness of greater than one mil. Once the molybdenum/manganese screen has been applied, a metallization scheme such as nickel/gold is formed on the molybdenum/manganese screen. The screen layer is necessary in the thick film process because aluminum nitride typically has very poor adhesion properties to metals. The necessity of this thick screen film is undesirable because it increases the size of the semiconductor package and leads to severe problems in electrical performance. Further, the molybdenum/manganese screen is not resistive and accordingly, no resistive properties can be derived therefrom.
Various thin film processes using aluminum nitride substrates have also been investigated. Once such thin film process includes sputtering a titanium or tungsten layer on an aluminum nitride substrate and then firing the film. Reactive sputtering of titanium or tungsten may also be employed. The reactive sputtering is typically performed in a controlled chamber so that the temperature and gases are such that a nitride is formed between the aluminum nitride substrate and the metal film. This nitride allows adhesion between the substrate and the film. A disadvantage with this method is that neither titanium or tungsten are resistive materials and therefore, many desired resistive properties are absent.
Accordingly, it would be highly desirable to have a process for fabricating resistive conductive patterns on aluminum nitride substrates which overcomes the limitations of the prior art.